Fibre reinforcing composites

ABSTRACT

Fibre reinforced composites of cementitious materials or gypsum are reinforced with pre-combined mixtures of strong reinforcing fibres e.g., glass or steel, and water absorbent fibres, e.g., cotton. Examples of composites are cement pipes, wall boards, etc.

This is a division of application Ser. No. 343,863, filed Mar. 22, 1973,now U.S. Pat. No. 3,903,879.

FIELD OF THE INVENTION

This invention relates to fibre reinforced composites of the type inwhich gypsum or inorganic portland cementitious matrix materials arereinforced by fibres, and to methods for their preparation.

BRIEF DESCRIPTION OF THE PRIOR ART

Fibre reinforced composites of cementitious materials with variousstrength - conferring fibres are known. Examples are asbestos-cementsheets, and steel fibre - and steel wire - reinforced composites knownas ferro-cement structures.

Several methods are known for preparing such cementitious composites. Afirst method is simple mixing of short reinforcing fibres (asbestos orchopped glass, for example), with the matrix material in fluid form(e.g. a slurry of portland cement paste), and subsequent hardening. Insuch a method, the fibres are generally damaged during mixing. Thestrength of the composite is low, because the fibres are randomlyoriented. The amount of chopped fibres which can be added is limited,since the fibres greatly increase the viscosity of the matrix material,when it is in its liquid form. As this viscosity increases, it becomesincreasingly difficult properly to coat the fibres. With cementitiousslurries e.g. portland cement paste, increasing the dilution of theslurry reduces the viscosity, but at the same time reduces the strengthof the hardened material. One can concentrate the slurry after coatingand before hardening, e.g. by drawing excess moisture under vacuum, butadds an expensive process step.

In a second method, reinforcing fibres, filaments, wires or rods areshaped, laid up or woven over on or in a mold armature or form andimpregnated with fluid matrix materials, for example in the productionof ferro-cement boat hulls or steel reinforced concrete structures. Sucha process is time-consuming and expensive, involving manual operations.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide novel composites ofcementitious or gypsum matrix materials and fibres.

A further object is to provide new methods of making composites ofcementitious or gypsum matrix materials and fibres, which are versatileand economical.

A further object is to provide novel fibre combinations useful inreinforcing such composites.

Other objects will be apparent from the following description andspecification.

Briefly, these objects are accomplished in accordance with the inventionby the provision of fibre reinforced composites of cementitious matrixmaterials and gypsum matrix materials of improved strength and otherproperties, having precombined absorbent fibres and reinforcing fibres,as the reinforcement.

By "cementitious matrix materials" as used herein is meant inorganiccement based hardenable materials. The term does not include gypsumbased materials, to which different considerations apply, and which areseparately identified herein.

The absorbent carrier fibres used in the present invention are generallytextile fibres, for example natural fibres such as cotton, wool, hemp,abaca, silk, sisal, jute, flax and cellulose (paper or wood),regenerated fibres such as viscose rayon, cuprammonium rayon andcellulose acetate or absorbent synthetic fibres such as nylon 66,polyacrylonitrile, or polyvinyl alcohol and absorbent types ofpolyesters or polyacrylics. Preferred among such fibres are cotton,wool, cellulose, viscous rayon and cuprammonium rayon, with the mostpreferred being cotton and rayon, generally on account of cheapness anddesirable absorbence. The terms absorbent and non-absorbent used hereinrefer to the behaviour of fibres toward aqueous cementitious slurries.Absorbent fibres have a moisture regain under standard conditions (65%relative humidity at 20° C) above about 21/2%.

The reinforcing fibres used in the present invention are non absorbentand those commonly used in preparing cementitious composites, andinclude fibres of glass, steel, carbon, boron, copper, brass, aluminumand its alloys, asbestos and silicon compounds, as well asnon-absorbent, strong types of synthetic polymeric fibres such asnon-absorbent polyamides, non-absorbent polyesters, non-absorbentpolyacrylics, polyolefins such as polyethylene and polyurethanes.Preferred are fibres of glass, steel, carbon, polyethylene andpolypropylene. The reinforcing action is believed derived largely fromthe reinforcing fibres alone. However, the presence of the absorbentcarrier fibres appears to facilitate coating or impregnating the mixtureof fibres with the matrix material, by providing the necessary adhesiontherebetween.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

It is preferred to unite the absorbent fibres and reinforcing fibresprior to application of the matrix material, to make a fibrecombination. This combination can be a single reinforcing fibre twistedwith an absorbent fibre, or a multi-ply yarn of several absorbent andreinforcing fibres spun together. The reinforcing fibres and carrierfibres can be inter-woven together, into a mesh, cloth or tape. Theaforementioned multi-ply yarn or twisted fibre combination can be woveninto a cloth, mesh or tape. Meshes etc., with fibres spaces at intervalsof about 4 to about 30 ends per inch are preferred. These fibrecombinations can be easily coated or impregnated with matrix materialssuch as slurries of portland cement paste or gypsum, even though thesematerials generally do not adhere to reinforcing fibres alone. The term"reinforcing fibres" is intended to include the materials in the formsof whiskers, flakes, ribbons, filaments, strands, rovings, etc.

It has been found that, in addition to assisting in pickup of cement orgypsum pastes, the absorbent fibres in the composites of this inventionincrease the interlaminar bond strength significantly. They thusincrease the bond strength between successive layers of impregnatedfibres.

Once the fibre combinations impregnated with matrix material have beenprepared, they can be used in a variety of ways to provide high qualitycomposite materials in an efficient manner. They can be chopped intosmall lengths before or after impregnation with the matrix material andmolded in accordance with the first process described, although there islittle advantage in such a process, under most circumstances. They canbe molded into composite shapes in the form of cloths or meshes, inaccordance with the second process described, where the matrix materialis impregnated before or after molding. Preferably, however, they areused for molding composites by winding or arranging the impregnatedcombination into the desired shapes and hardening.

An application of this invention is in steel wire reinforced concrete.In normal process, such structures are made by mixing about 2% by weightof chopped steel fibres, about 10 mils diameter and 1 inch in length, inordinary portland cement concrete. There is a tendency for fibres inthis process to become damaged during mixing. In accordance with thepresent invention, one can make combination tapes of high strength steelwires and cotton yarn, by weaving on a shuttle type loom. Use of suchtapes in steel wire reinforced concrete gives superior properties. Aspreviously disclosed, one can substitute other strong fibres, such asglass, etc., for the steel.

Having prepared preformed tapes of interwoven strong fibres andreinforcing fibres, one can then impregnate these with aqueouscementitious slurries. Then the impregnated tapes can be laid up in thedesired manner to fabricate finished articles by drying and hardeningthe cementitions matrix materials.

For example, the impregnated tapes may be wound spirally over acylindrical mandrel, to form fibre reinforced cement pipes. Layers ofthe impregnated tapes may be built up within shaped, removable forms ormoulds, to make articles such as fibre-cement boat hulls, orfibre-cement building blocks, wallboards, tanks, etc. Methods offabricating articles somewhat resemble known techniques for making glassfibre resin reinforced items, where glass fibre tapes are impregnatedwith resins, put into molds and hardened, or laid side by side onpreformed structures and hardened in situ or wound onto preformedstructures and hardened in situ. The presence of the absorbent fibres inthe composites of this invention ensures pickup of sufficientcementitious slurry to form satisfactory finished articles.

Instead of impregnating the pre-woven tapes, one can lay up the tapes inthe mold or where else they are to be fabricated, and then pour cementslurry over them. A further alternative is to apply a mixture ofcementitious matrix materials and volatile solvents, e.g. ethyl alcoholto the webs, and after evaporation of the volatile solvents lay them inthe mold or elsewhere and apply water to them. Cement slurry can beapplied by brush or rolling onto the tapes, instead of by dipping.

As noted, it is preferred to use combinations of fibres which havepreviously been mechanically combined such as interwoven into tapes orsheets. It is also preferred to use either steel or thermoplasticorganic polymer coated glass fibres as the reinforcing fibres. Uncoatedglass is subject to attack by alkalis present in the cement. If desired,coated or galvanized steel fibres can also be used, so as to avoid anycorrosion of such fibres at the surfaces of finished articles.

A preferred reinforcing fibre in glass fibre yarn coated with athermoplastic polymer, prior to forming the fibre combination.

This thermoplastic coating performs functions. It enhances the adhesionbetween the cement on gypsum and glass fibres, to increase the flexuralstrength of the composite. It has been found that the shear bondstrength between polyethylene coated glass fibre and gypsum is about 6times that between uncoated glass fibre and gypsum. It protects theglass fibre from physical damage during handling, weaving, cutting,packaging etc., and from damage by water, solvents or chemical attack.Uncoated glass yarn is so sensitive that it can easily lose 50% of itsstrength by damage during slight handling. The coating also assists instabilising the form of the web, by increasing the surface friction ofthe glass fibres and by producing "weldable" points of intersection offibres. The use of uncoated glass fibres and cotton, etc., would notproduce stable webs with ordinary gauze stitch, because the lack offriction between fibres would cause displacement of fibres. One way ofavoiding this is to employ a leno stitch at the fibre intersections, butthis cannot be used with glass fibres because normal glass fibres lackthe necessary degree of flexibility to allow leno stitching withoutbreaking. With the coated glass fibres in the present invention, simplegauze stitching is satisfactory. In addition, bonds can be formed at thefibre intersections by heat softening, e.g. by passing the web throughheated press rollers. This keeps the web intact and prevents pull-out ofindividual fibres. The surface friction can be controlled by changingthe roughness of the coating on application.

Examples of suitable coatings are polyethylene, polyvinyl chloride,polypropylene, nylon, polystyrene, silicones and other inert polymericmaterials. Polyvinyl Chloride, polyethylene, polypropylene and nylon arepreferred.

Methods of preparing such resin coated glass fibres include extrusionmethods and fluidized bed coatings. In extrusion methods, the fibres tobe coated are pulled through the coating polymer which is in fluid form,and then through a die opening. The polymer may be heat softened, or insolution in a solvent, or in a dispersion in water. In this method,however, there is a practical minimum of thickness of resin coatingwhich can be achieved. In the fluidized bed method, the cold fibre ispassed through a fluidized bed of powdered polymer, and then through aheater to melt the polymer onto the fibre. This allows very thincoatings to be applied. The roughness of the coating can be controlledby pre-soaking the fibre in a liquid such as aqueous detergent solution.

The thickness of the coating applied is largely dictated by practicalconsiderations. It should be sufficient to ensure continuous coatingover the major proportion of the entire surfaces of the fibres and sothat heat softening and pressing will not cause significant voids in thecoating to form. On the other hand, too thick a coating makes animpractically bulky fibre and increases the costs without achievingtechnical benefits. Coatings of thickness about 0.3 to 5 mils arenormally satisfactory.

Cementitious or gypsum materials can be further strengthened by addingpolymer latices to the paste, or by subsequently impregnating thehardened material with a monomer. Suitable latices include those ofvinyl acetate polymers and copolymers, styrene polymers and copolymers,vinyl chloride polymers and copolymers, and polyacrylates. Elastomerlatices, e.g. those of butadiene polymers, may also be used. They aresuitably used in amounts of 0.15 to 0.25 parts polymer per part byweight of cement or gypsum. For impregnating, methyl methacrylate is asuitable monomer. The hard dry composite may be soaked in the monomer,or vacuum impregnated therewith, and then heated or irradiated to causepolymerization.

The additional strength conferred by the fibre combinations according tothe invention means that thinner, lighter cementitious structures can beprepared than previously, without loss of strength. Ferro-cement boats,for example, can be made economical in this manner.

Instead of interweaving the fibres to make a web or tape, they can beformed into a felt. This is done using, for example, mixtures of choppedglass fibres and cotton, paper or wool. These are mixed with a suitableadhesive, e.g. polyvinyl alcohol solution and laid as a felt. Such feltscan then be impregnated with cementitious matrix material in fluid form,and allowed to harden. Whilst such composites are not as strong as thosebased on pre-woven tapes, they are cheaper, and allow use of very cheapfibrous materials e.g. paper fibres.

The composites of the invention can also be used to effect repairs toconcrete structures, e.g. swimming pools. Combination tapes of theinvention can be impregnated with cementitious matrix materialspreferably latex modified cement, and smoothed on the concrete surface,in one or more layers.

The following are illustrative specific examples.

EXAMPLE 1

Steel wire reinforced concrete was prepared in accordance with thepresent invention, as follows.

Four inch wide tapes of high strength steel wires and cotton yarn werewoven on a shuttle type loom, to give a fibre combination. The steelwire had a diameter of 6 mils and a strength of about 400,000 p.s.i. Thecotton used was preshrunk two ply No. 30 bleached yarn. Ten ends perinch for each fibre was used in both the longitudinal (warp) and cross(weft) directions.

The tapes, after this preparation, were passed through a portland cementmortar slurry consisting of one part water, two parts cement, threeparts sand by weight, and then were wound into a cylindrical mold. Themold was then placed in a standard moisture curing room for 1 month. Thecylindrical shell or reinforced concrete was then removed from the mold,and was found to exhibit about three times the flexural strength, andabout 20 to 30 times the facture toughness which could be obtained frommortar reinforced with equal weight fractions of chopped steel wires.

Similar shell type structure could be produced by winding the wire intothe same mold, and then trowelling in the mortar, in accordance withpreviously known processes. However, the trowelling operation isextremely time-consuming, is likely to damage the fibres, and leads toan uneven wall thickness in the structure. In accordance with thepresent invention, a combination steel wire/cotton yarn tape is usedwhich is impregnated before application of the mortar. This provideshigh quality structure at low fabrication cost, and the steel wires canbe aligned in any direction desired. In addition, the wall thickness ofthe structure can be controlled within close tolerances.

The fabrication technique can be used in the production of shellstructures such as grain bins, or crude oil tanks. It can also be usedto provide modular housing units.

EXAMPLE 2

Gypsum board was made using fibre combinations of the invention.

A glass/cotton tape was prepared, using a PVC coated glass yarn,pre-twisted with 2 ply No. 50 bleached cotton, woven into a 6 inch widetape with 12 ends per inch composite yarn in the longitudinal (warp)direction and 22 ends per inch in the transverse (weft) direction. Thegypsum was plaster of paris β-hemihydrate, and contained 1% Keratin toretard setting up of the gypsum. A slurry was made using 0.55 partswarter per part gypsum.

Pieces of tape 6 inches by 12 inches were cut, dipped in the slurry andplaced on a release coated surface, in a pile. The pile was pressed toremove air bubbles. This produced a 4- layer gypsum/fibre board ofthickness about 3/8 inch, which was allowed to harden for 1 week. Acontrol was also prepared, of the same thickness, usng no fibres. Thesamples were then tested for strength with specimens cut out in thelengthwise direction. Modulus of rupture and fracture toughness weredetermined using an Instron Universal Tester, by three point bending.Lateral nail resistance was tested by driving in a 0.116 inch diameternail, 1/2 inch from the sample edge and pulling it out with the Instrontester. The results were as follows:

    ______________________________________                                                     Modulus of                                                                            Fracture  Lateral                                                     rupture toughness nail                                                        (p.s.i.)                                                                              (in.lb.)  Resistance                                     ______________________________________                                        Reinforced gypsum board                                                                      1360      36.0      120.0                                      Control gypsum board                                                                          850       1.9       14.0                                      ______________________________________                                    

The fibre reinforced boards can be used in place of plywood in manyapplications. The process of this invention gives strong, tough, highquality wall or floor boards efficiently and economically. They can bestrengthened even further by incorpoation in the gypsum of a polymerlatex, such as a polyvinyl or polyvinylidene chloride latex.

EXAMPLE 3

Glass reinforced high alumina cement composites were prepared, using 12inch wide tapes of composited yarn of pretwisted glass yarn and 2-plyNo. 30 bleached cotton. The tape was woven in a standard shuttle typetextile loom, to give 22 ends per inch in both directions.

Pieces of tape 6 by 12 inches were impregnated with high alumina cementslurry, containing 0.55 parts water per part cement. The sheets werepiled 16 high on a release coated surface to give a composite ofthickness 3/16 inch. A control sample of the same thickness withoutfibrous layer reinforcement, was prepared of the same cement. Both wereleft to dry and harden for 1 week, then tested as in Example 3, with thefollowing results.

    ______________________________________                                                     Modulus of                                                                              Fracture                                                            Rupture(psi)                                                                            Toughness (in lbs.)                                    ______________________________________                                        Fibre reinforced composite                                                                   3960        62.1                                               Control         550         2.3                                               ______________________________________                                    

High alumina cement is desirable for use with uncoated glass fibres,since it does not release alkali. With regular cement, coated glass ispreferred, for resisting alkali.

EXAMPLE 4

Steel wire reinforced mortar was prepared according to the invention.

Alternate layers of fine steel wire mesh and leno stitched cottom gauzewere impregnated with latex modified portland cement paste. The latexwas a styrene-butadiene latex, used in an amount of 0-15 parts latexsolids per part cement. 8 layers of steel wire and 9 pieces impregnatedcotton were piled alternately to make a board about 3/8 inch thick.Whilst the steel wire mesh was run through the cement paste also, itpicked up virtually no cement. Controls of unreinforced cement andunreinforced latex impregnated cement were made to the same thickness,and all specimens hardened for one week. Flexural strength tests wereconducted as in Example 3, with the following results:

    ______________________________________                                                             Fracture  Strain to                                                  Modulus of                                                                             Toughness First                                                      Rupture(psi)                                                                           (lb.in)   Crack(%)                                       ______________________________________                                        Reinforced latex                                                                            3180       38.2      0.12                                        modified cement                                                              Latex modified cement                                                                       640        3.5       0.10                                        control                                                                      cement control                                                                              520        2.0       0.03                                       ______________________________________                                    

A further specific embodiment of the invention is a medical cast.Conventionally, casts are made by impregnating a cotton bandage withgypsum slurry and wrapping the bandage around a fracture site. Sincecotton fibres provide weak reinforcement, the cast has to be bulky andheavy, to have the necessary strength. Whilst proposals have been madeto use stronger fibres, such as glass fibres, these have not found wideacceptance. Once of the problems is lack of adhesion of gypsum slurriesto fibres such as glass.

The casts according to the present invention use bandages of combinedreinforcing fibres, preferably glass, and absorbent fibres, preferablycotton. The glass fibres are coated with a thermoplastic vinyl resin, inaccordance with the previous description, prior to forming thecombination bandage. In addition to providing advantages previouslydiscussed, when used in casts the coating prevents irritation ofsurgeon's or patient's skin. For use in casts, polyethylene andpolyvinyl chloride coatings are preferred.

Bandages for casts according to this invention preferably are interwovenof absorbent (e.g. cotton) fibres in the longitudinal direction andcoated glass fibres in the transverse direction. For proper application,it is preferred that the bandage be stretchable longitudinally, which isnot achieved with coated glass fibres, unless special, very fine glassfibres are used. Conversely, inextensibility as provided by glass fibresis desirable in the transverse direction for fracture immobilization.Mixtures of cotton and glass fibres can alternatively be used in thetransverse direction, the fibres being either pre-combined e.g. byintertwining, or separately woven into the bandage. The preferredinterwoven bandages have the additional but subjective advantage ofacceptable feel and handle to the surgeon, similar to well establishedcotton bandages.

The following illustrates a cast according to the invention.

EXAMPLE 5

A glass reinforced plaster cast, for immobilizing a fracture, wasprepared as follows. A bandage, 12 inches in width, was prepared out ofcotton and glass yarn 8 mil thick. This combination bandage was woven ona shuttle type loom. Prior to weaving the glass yarn was coated with avinyl plastic, specifically polyvinyl chloride, by an extrusion process.After coating, the glass fibres had a diameter of 11 mils and a strengthof about 200,000 psi. The cotton used was pre-shunk two ply No. 30bleached yarn. In the longitudinal or warp direction of the bandage,cotton only was used, with 22 ends per inch. In the cross or weftdirection, the coated glass fibre yarn was used alone, with 16 ends perinch.

The composite bandage thus made was impregnated with a gypsum slurryeasily, and slit to a width of about 4 inches. It could be applied to afracture site as easily as a conventional cotton bandage. Laboratorytesting showed that the glass reinforced plaster cast according to theinvention had approximately five times the flexural strength, twice theelastic modulus and 11/2 times the fracture toughness of conventionalplaster casts in the direction of the glass reinforcement. Further,medical trials showed that the glass reinforced cast did not causeirritation of the patient, was much more durable, and could be removedby conventional means.

In application, the surgeon can wind the bandage in both clockwise andcounterclockwise directions around the fracture limb, to provide glassfibres which are aligned in two directions slightly offset from thelongitudinal direction of the cast.

What we claim as our invention is:
 1. A fibre reinforced gypsumcomposite comprising gypsum matrix material having embedded therein awoven tape, said woven tape comprising an interweave of absorbent fibreswith reinforcing glass fibres, said reinforcing glass fibres being glassyarn coated with a thermoplastic organic polymer, said polymer beingselected from the group consisting of polyethylene, polyvinyl chloride,polypropylene, nylon and polystyrene.
 2. The fibre reinforced gypsumcomposite of claim 1 wherein the thermoplastic organic polymer ispolyethylene or polyvinyl chloride.
 3. The fibre reinforced gypsumcomposite of claim 2 wherein the absorbent fibre is cotton or rayon. 4.The fibre reinforced gypsum composite of claim 3 wherein the tape hasfibres spaced at intervals of about 4 to about 30 ends per inch.
 5. Thefibre reinforced gypsum composite of claim 3 wherein the coated glassyarn and the absorbent fibre are pre-twisted together into a compositeyarn, and the composite yarn is used as the fibres in at least onedirection of weave of said tape.